The present invention relates to a method of forming an article via injection of plastics materials into a mould.
It should be emphasised that in this specification, including the claims, the meaning of the term xe2x80x9cblowing agentxe2x80x9d encompasses both xe2x80x9cchemical blowing agentsxe2x80x9d and xe2x80x9cphysical blowing agentsxe2x80x9d. A chemical blowing agent comprises at lest one compound which breaks down in molten plastics material to mix gas into the material. A physical blowing agent is a gas or a liquidxe2x80x94at room temperature and pressurexe2x80x94or mixed with the molten material. Chemical blowing agents are added to the plastics material in the hopper of material fed to an injection moulding machine""s plasticising screw. Physical blowing agents can be mixed with the plastics material as described in my UK patent applications Nos. 9702977.1 and 9706682.3, respectively dated Feb. 13 and Apr. 2, 1997. These are still unpublished. However, the mechanism for producing a mixture of blowing agent and plastics material forms no part of this invention, which is concerned with production of articles using such mixtures. Generally, the blowing agent as such will add negligibly to the volume1 of the material when being processed within an injection moulding machine due to the elevated pressure within the machine. However, when the pressure is released to substantially lower pressure, such as room pressure, and the plastics material is still molten, the blowing agent acts to expand the material by the blowing voids of varying sizes in the material.
1 Chemical blowing agents are usually bound into a carrier of other plastics material which has its own bulk. 
A recognised problem, in injection moulding from plastics materials of articles having thin walls in particular, is that at marked differences in cross-section, differential shrinkage causes blemishes in the finished surface of the article.
Despite this problem, many new designs of injection moulded articles could be feasible if marked changes in cross-section were possible, without shrinkage blemishes.
The following applications describe my basic invention having the object of providing an improved method based on injection moulding and facilitating the production of substantial changes in cross-section:
British Patent Application No. 9514674.2
U.S. Patent Application No. 60/017,386
International Patent Application No. PCT/GB96/01706, now published under No WO 97/03800.
In these applications I describe and/or claim (as proposed to be amended):
A method of forming an article via injection of plastics material into a mould, the finish formed article having thin wall portion(s) and thick wall portion(s), the thick wall portion(s) being at least partially foamed, the method consisting in the steps of:
providing a mould tool defining in its closed state, between its cavity part and its core part, narrow gap portion(s) whose mould part gap is to be substantially reproduced in the thin wall portion(s) of the article and wide gap portion(s) whose mould part gap is less than the thickness of the thick wall portion(s) of the finish formed article;
closing the mould tool to define the narrow and wide gap portions;
injecting a plastics material mixture comprising a basic polymer and a foam producing additive into the mould tool;
allowing the plastics material mixture to at least substantially solidify in the narrow gap portions of the mould tool to produce the thin wall portions of the finish formed article;
withdrawing at least a portion of one part of the mould tool from the other part before the plastics material mixture has at least substantially solidified in the wide gap portion(s) of the mould tool to allow the mixture to expand by foaming and form at least some of the thick wall portion(s) of the finish formed article; and
ejecting the article from the mould tool.
This method is hereinafter referred to as xe2x80x9cMy Original Methodxe2x80x9d.